I would try flying without the cowl. If it does not overheat then I suggest that you you extend the slot around the carb. upwards towards the cylinder head. You might also try internal baffles inside the cowl to force the airflow around the exhaust side of the cylinder and over the head fins.

You need to create airflow over the engine fins and this can be bit hit and miss. I think that large cut out on the rh side of the cowl [ looking from the rear] may be counter productive.